With the renewed and ever expanding interest in Stirling engines, efforts have been made to continually improve upon their design. Basic Stirling engine principals of operations are set forth in a text entitled "Stirling Engines" by G. Walker, 1st Edition, 1980. Essentially in this regard, a Stirling engine operates on the principal of heating and cooling a working fluid (gas), with the expansion and compression of the gas utilized to perform useful work. A variety of designs are illustrated in the aforenoted text with their attendant advantages.
A typical Stirling cycle consists of a contained volume divided into the following adjacent regions: compression (or cold) space, cooler, regenerator, heater and expansion (or hot) space. In actual construction though these spaces are necessarily connected by ineffective regions or connecting ducts. Thermodynamically, it is less severe when occurring between the regions where working fluid is hot and less dense than when occurring in the cooler regions where the working fluid is more dense. In most cases, the largest connecting volumes are between heater and expansion space, and cooler and cold space. Of these two, the cold duct is the most disadvantageous to power density and efficiency, so it is, an object of this design to minimize that volume.
In addition, the majority of present Stirling engines utilize lighter-than-air gases such as hydrogen or helium as the working fluid due to their relatively high conductivity, high specific heat and low viscosity. However, a disadvantage of a lighter-than-air Stirling engine is that a fixed inventory of the gas is required and therefore also fairly completed sealing between the working spaces and ambient conditions. Current hydrogen and helium engines use a sliding seal on a rod between the pistons and the crossheads (which absorb side loads), to prevent oil leakage from the crankcase into the working space and working fluid leakage from working space to crankcase. Such an arrangement adds complexity, weight and volume to the engine.
Other designs envision the use of air as a working fluid. While such air-cycle engines avoid certain of the sealing requirements of the lighter-than-air engines and have other advantages to compensate for air's relatively poor fluid properties, a variety of design hurdles must be overcome, particularly providing an efficient power to weight ratio, since many of such air-cycle engines tend to be relatively heavy and need to be improved and simplified.
In either situation, air or lighter-than-air cycle Stirling engines, it is desirable to streamline them, simplifying their design and reducing their weight, while maintaining or improving their operating efficiencies.
While many of the prior designs of Stirling engines have proven acceptable in certain applications, there exists an ever present need to improve on such designs to provide a more efficient and less expensive engine.